I just upgraded from 2.7 -> 2.8.1 and found that a good number of my thumbnails (which I display next to posts) were not displaying. I traced this to strange behaviour in the core wp_get_attachment_thumb_url function itself which shows no overt changes between versions. What happens is that this function sometimes inserts absolute paths ('/home/.whatever/user/domain.com/img/') before the image URL and immediately after the domain. This is particularly odd as I no longer use these absolute paths (they are from old server configurations) and I cannot find them anywhere in the database.

Presently I am stripping the various paths out with str_replace but would really like to know what the root of this problem might be. Any ideas?